The present invention relates to an amplifier circuit for use in various electronic apparatus.
There are many types of negative-feedback amplifier circuits. Among them, an amplifier circuit employing an operational amplifier (hereinbelow, termed "OP amplifier") can produce only one output.
Prior to the present invention, the inventors studied a first system wherein an input signal is applied to the noninverting input terminal of an OP amplifier, a first feedback resistor is connected between the output terminal and the inverting input terminal, and a second feedback resistor is connected between the inverting input terminal and ground potential, thereby to set the closed-loop voltage gain of the OP amplifier by means of the first and second feedback resistors and to derive another output signal from the node of the first and second feedback resistors through an impedance converter of high input impedance and low output impedance (for example, an emitter follower circuit). A second system was also studied wherein another output signal is derived from the noninverting input terminal through the impedance converter.
The inventors' study, however, has revealed that the other output signals derived through the impedance converters such as emitter follower circuits have large distortions in both the above systems.